1. Field of the Invention
The present invention relates to a pickup feed mechanism which slidingly moves an optical pickup or the like which is used in various types of disc-shaped record medium driving units.
2. Related Art
A disc-shaped record medium driving unit has an optical pickup for reading out information recorded in a disc-shaped record medium like an optical disc. The pickup is located to be opposed to the front face of the disc-shaped record medium and can slide in a radial direction of the disc-shaped record medium.
To slidingly move the pickup in a radial direction of the disc-shaped record medium, there have been used feed mechanisms, for example, disclosed in Japanese Patent Application Laid-open No. H. 11-149728 or NO. H. 11-185407. The feed mechanism has a drive motor, a screw shaft rotationally driven by the motor, a nut moving with the pickup, and a resiliently urging means.
The motor rotates the screw shaft around the axis of the shaft. The screw shaft is defined like a rod extending in a direction and has a screw groove on an outer peripheral surface thereof. The nut is positioned to engage with the screw groove of the shaft. The nut can engage with and disengage from the screw shaft. The resiliently urging means pushes the nut against the screw shaft.
In the thus configured prior-art feed mechanism, the motor rotates the screw shaft so that the nut moves parallel to the axial direction of the screw shaft to transfer the pickup in a radial direction of the disc-shaped record medium.
In the prior-art feed mechanism, a runaway operation of the motor moves the nut to an end of the screw shaft, so that the screw groove of the screw shaft abuts against the screw groove of the nut. Thereby, the nut is pushed in a direction away from the screw shaft against a resilient force of the urging means, resulting in disengagement of the nut from the screw shaft.
The prior-art feed mechanism has a larger friction between the screw groove of the screw shaft and the nut when the resiliently urging means provides a comparatively stronger resilient force. Thus, the motor needs to provide a comparatively larger rotating torque, resulting in a larger size of the motor.
In the mean time, when the resiliently urging means has a comparatively smaller urging force, the friction force between the screw groove of the screw shaft and the nut tends to disengage the nut from the screw groove of the screw shaft. Thus, it happens that, during the rotation of the screw shaft, the nut is disengaged from the screw shaft not to move the pickup to a desired position.
That is, the prior-art feed mechanism tends to be disadvantageous for a flexible design of the urging force of the resiliently urging means and the rotating force of the motor.
In view of the problems described above, an object of the present invention is to provide a pickup feed mechanism allowing a flexible design thereof.
For achieving the object, a pickup feed mechanism according to the present invention includes:
a screw shaft having a screw groove and driven to turn,
an engagement piece moving parallel to an axis of a screw shaft, the engagement piece having a thread portion engaged with the screw groove of the screw shaft,
a pickup assembly moving parallel to the axis of the screw shaft together with the engagement piece, and
a bearing portion rotatively supporting the screw shaft, the bearing portion allowing the screw shaft to move in a direction away from the engagement piece.
Thus, a larger urging force for pushing the engagement piece against the screw shaft is unnecessary for surely engaging the engagement piece with the screw shaft. This does not require a larger sized driving unit for rotating the screw shaft.
Furthermore, when the rotation of the screw shaft moves the engagement piece to reach an end of the screw shaft, the screw shaft displaces in a direction apart from the engagement piece. Thus, the screw shaft can disengage from the engagement piece without applying an undesirable smaller or larger urging force against the screw shaft.
Accordingly, the urging force of the engagement piece against the screw shaft is normally determined. This is advantageous for a design of the driving mechanism.
Preferably, the engagement piece is movable toward and away from the screw shaft, and the mechanism further has a restrictor means preventing the engagement piece from disengaging from the screw shaft.
Thus, when the rotation of the screw shaft moves the engagement piece to reach an end of the screw shaft, the engagement piece and the screw shaft move apart from each other. Thus, the screw shaft can disengage from the engagement piece without applying an undesirable smaller or larger urging force against the screw shaft.
Moreover, in a normal operation, the restrictor means prevents the engagement piece from disengaging from the screw shaft. Thus, the resiliently urging means can have an appropriate urging force to keep the engagement of the screw shaft with the engagement piece so that the pickup assembly can surely move to a desired position.
Preferably, the mechanism further comprises a resiliently urging means abutting the engagement piece against the screw shaft. Thereby, the screw groove of the screw shaft can surely engage with the engagement piece.
Preferably, the restrictor means moves together with the pickup assembly, and the restrictor means is a restrictor piece which is located in an outer peripheral side of the screw shaft such that the engagement piece keeps a distance from the screw shaft not to disengage from the screw shaft.
The restrictor piece serves to keep the engagement of the screw shaft and the engagement piece with an appropriate urging force, so that the pickup assembly can more surely move to a desired position.
Preferably, the bearing portion has a first bearing rotatively supporting one end of the screw shaft and a second bearing rotatively supporting the other end of the screw shaft, the first bearing having a recess receiving the one end of the screw shaft and a tapered surface provided at an outer periphery of the recess, the tapered surface inclining outward to be opposed to the other end of the screw shaft, the second bearing resiliently urging the other end of the screw shaft toward the one end and allowing the one end of the screw shaft to disengage from the recess.
Thus, when the rotation of the screw shaft moves the engagement piece to an end of the screw shaft, the engagement piece pushes the screw shaft so that the one end of the screw shaft can move along the tapered surface. That is, the screw shaft displaces along the tapered surface in a direction apart from the engagement piece.
Thus, the screw shaft can more surely disengage from the engagement piece without applying an undesirable smaller or larger urging force against the screw shaft.
Preferably, the second bearing has a support extension and a spring piece, the other end of the screw shaft rotatively supported between the support extension and the spring piece, the spring piece having an end face urging the other end of the screw shaft toward the one end of the screw shaft.
This prevents the screw shaft from undesirably deviating in the axial direction, except when the engagement piece moves apart from the screw shaft at an end of the screw shaft.